Absorber and Distillation Column Control with WirelessHART Measurement Devices

We’re now inside of a week from the Emerson Exchange conference in Orlando. One of the sessions I definitely plan on visiting is one by Dr. Frank Seibert of the University of Texas at Austin and Emerson technologists, Terry Blevins and Mark Nixon. Their presentation, Stripper and Absorber Control Using WirelessHART Transmitters will be held Wed, 9/30 at 3:15pm in the Osceola 1 room and repeated Thurs, 10/1 at 11:00am in the Naples 3 room.

Dr. Seibert is the Technical Manager of the Separations Research Program at UT. This cooperative industry/university program performs fundamental research of interest to chemical, biotechnological, petroleum refining, gas processing, pharmaceutical, and food companies. One key area of focus is CO2 removal from stack gas. An absorption system using air/carbon dioxide/amine solvent is used to remove the CO2. The stack gas is sent to the Absorber unit to absorb the CO2 and then a distillation column / stripper is used to strip the CO2 from the amine stream.

WirelessHART pressure and flow transmitters were installed to test control across WirelessHART. WirelessHART pH transmitters were installed to study the CO2 removal from stack gases. WirelessHART multi-element temperature transmitters were installed to monitor the absorber and stripper temperatures. Dr. Seibert wanted the flexibility to work on the absorber and stripper units without connecting and disconnecting cables. This helped reduce downtime and maintenance. The elimination of grounding issues which impacts the reliability and accuracy of the pH and temperature measurement was another advantage of this approach.

Traditional PID control relies on the underlying assumption that the PID controller executes on a periodic basis, mainly due to the calculated reset and rate action. WirelessHART devices conserve energy by reporting on an exception basis–only when the value changes by a set level. Working with Terry, Mark and the Emerson technologists, a method was developed in the DeltaV PID control algorithm to support control from WirelessHART transmitters.

The team compared the control performance using the modified PID algorithm and WirelessHART instruments versus a standard DeltaV PID algorithm with wired devices. Column pressure control and heater stream flow control using WirelessHART transmitters and DeltaV modified PID option provide the same dynamic response and comparable performance to that achieved using wired transmitters and traditional PID. An interesting finding was that this comparable performance was achieved when the number of measured samples was reduced by a factor of 10 for the flow control and by a factor of 6 for the pressure control.

More importantly, the cost to relocate wiring has been eliminated through the installation of WirelessHART transmitters for stripper steam flow, column pressure, column temperatures, and pH. With no grounding issues to contend with, the improved accuracy and reliability of the pH and temperature measurements can improve stripper and absorber operation.

This modified PID algorithm for inputs from non-periodic WirelessHART measurement devices is being incorporated into the DeltaV v11 release.

Update and bump: Here are some additional notes live from the presentation. Dr. Seibert noted that 70-80% of their time was in moving and reusing instrumentation. The work is done on skid mounted systems which will be used in actual applications if the research proves successful.

Terry added that simulation was used for the checkout of the flow and pressure loops to verify operation before the loops were commissioned on-site at UT. He noted that tuning of the changed PID algorithm is identical from an instrument tech’s perspective.

Posted Wednesday, September 30th, 2009 under Wireless.

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